Electrolyte salts for use in electrochemical cells, e.g., lithium or lithium ion batteries, must exhibit good ionic conductivity and electrochemical, thermal, and chemical stability. In addition, the components of the electrochemical cell must be stable towards the electrolyte. Stability concerns are particularly acute in the case of electrochemical cells having aluminum cathode current collectors because aluminum is susceptible to corrosion.
Among the known electrolyte salts, lithium bis(trifluoromethanesulfonyl)imide ((CF.sub.3 SO.sub.2).sub.2 N.sup.- Li.sup.+) has good conductivity and stability, but is highly corrosive toward aluminum at potentials above 3 V(vs Li/Li.sup.+). LiPF.sub.6 has excellent conductivity and is noncorrosive, but is thermally and hydrolytically unstable. LiO.sub.3 SCF.sub.3 (also called lithium triflate) has good thermal and chemical stability, but has low conductivity and is also highly corrosive toward aluminum at positive electrode potentials above 3V(vs Li/Li.sup.+).
Indeed, the corrosion of aluminum at potentials above 3 V in electrolytes containing lithium triflate or lithium bis(trifluoromethanesulfonyl)imide is so severe as to make these salts of little use for applications in the more advanced, high voltage cells, especially rechargeable cells. Thus, the use of presently-available electrolyte salts in high voltage lithium or lithium-ion cells has resulted in batteries having suboptimal performance characteristics such as restricted operating temperature ranges, limited discharge/charge rates and inadequate cycling performance, particularly when aluminum components are used.